CPAP treatment of OSA, a form of Noninvasive Positive Pressure Ventilation (NIPPV), involves the delivery of a pressurized breathable gas, usually air, to a patient's airways using a conduit and a patient interface, for example, a mask. Gas pressure employed for a CPAP typically range from 4 cm H2O to 28 cm H2O, at flow rates of up to 180 L/min (measured at the patient interface), depending on patient requirements. The pressurized gas acts as a pneumatic splint for the patient's airway, preventing airway collapse, especially during the inspiratory phase of respiration.
CPAP machines including an airflow generator for supplying pressurized air to the patient are known, and over recent years there has been commercial incentive for more compact CPAP machines. However, in seeking to reduce the size of the CPAP machines there has been a trade-off between reduced size on the one hand and reduced performance on the other.
The advantages of incorporating humidification of the air supply to a patient are known, and CPAP machines are known which incorporate humidifying devices, either separately from the flow generator or integrated therewith. An example of an integrated flow generator/humidifier is the ResMed® S7 sold by the assignee of the present application. An example of a humidifier which is separately provided to be connectable to a flow generator is disclosed in U.S. Patent Application Publication 2008/0072900 A1, the entire contents of which are incorporated herein by reference.
It is known to provide a heating unit, such as a heating plate, to a humidifier to increase the amount of water vapor in the flow of breathable gas. Reducing the size of CPAP machines, including humidifiers, has led to a decrease in the size of water containers making it more difficult to provide humidification of the air supply during the entirety of the patient's sleep cycle. The reduction in the size of humidifier tubs results in a decrease in the surface area of the water exposed to the flow of air provided by the flow generator. This creates problems in maintaining a sufficient moisture pickup by the airflow passing through the tub and requires that the flow generator motor run faster, which produces more noise. The integration of humidifiers with flow generators also makes it more difficult to clean the water container of the humidifier.